(1) Field of the Invention
The present invention generally relates to a lead frame and a production method for producing a semiconductor device using the lead frame, and more particularly to a lead frame in which tie bars connecting leads can be removed in a resin encapsulation process for a semiconductor device and a production method for producing a semiconductor device using the lead frame.
A semiconductor device (hereinafter referred to as an IC) is produced using a lead frame. The lead frame has tie bars connecting leads to prevent resin from leaking from molding dies in a resin encapsulation process in which a semiconductor chip is molded by resin. Thus, after the resin encapsulation process for this type of semiconductor device, the tie bars must be cut off from the lead frame. It is preferable for the tie bars to be precisely removed from the lead frame.
(2) Description of the Related Art
An IC using a conventional lead frame is encapsulated by resin as shown in FIG. 1. Referring to FIG. 1, a lead frame 20 projects from both sides of a package 10 made of resin. The lead frame 20 has leads 22 arranged at predetermined intervals and tie bars 21 connecting the leads 22. In each space 15 surrounded by a lead 22, a tie bar 21 and the side surface of the package 10, resin is leaked from the cavity of the molding dies and is hardened, so that resin burr(dam burr made of a thin resin film) is formed in the resin encapsulation process.
After the resin encapsulation process, the tie bars 21 are cut off by a punching machine as shown in the cross-sectional view of FIG. 2, taken in a plane along line 2--2 in FIG. 1, so that the leads 22 are separated from each other. That is, the lead frame 20 set on a lower die 32 is punched by a punching die 31 so that the tie bars 21 are removed from the lead frame 20. In the punching machine, the conveying of the lead frame 20 and a punching action are alternately performed so that a predetermined number of the tie bars 21 are removed from the lead frame 20 in each punching action.
In a case, as has been described above, where the tie bars 22 are cut off from the lead frame after the resin encapsulation process, the tie bars 21 may not be precisely cut off due to the nonuniformity of thermal contraction of the lead frame 20 and/or a discrepancy between the lead frame 20 and the punching die in the punching machine. Increasing the integration degree of the IC so that the intervals at which leads are arranged is decreased, it is difficult to precisely cut off the tie bars 21. Further, if the discrepancy between the punching die and the lead frame occurs, a part of each lead is punched as shown in FIG. 3 so that the shape of each lead is deteriorated. If the shape of each lead is greatly deteriorated, it is determined that the IC is a defective device.
In addition, if the lead frame 20 is not set at a regular position in the molding die in the resin encapsulation process, a relative discrepancy between the lead frame 20 and the package 10 occurs, so that the tie bars 21 are positioned close to the side surfaces of the package 10. If the tie bars 21 are cut off by the punching machine in this state, the side surfaces of the package 10 may be damaged by the punching die, so that it is determined that the IC is a defective device.
Further, to enable tie bars of the lead frame to be easily cut off, the lead frame as disclosed in Japanese Laid Open Patent Application No.1-175250 has been proposed. In this lead frame, tie bars are formed so as to be pushed out from the surface of the leads in a direction of the thickness thereof by a distance equal to one third of the thickness of the lead frame. In the encapsulation process, when the lead frame is clamped by the molding dies, the tie bars are pushed back. As a result, a part of the border portion between each tie bar and a lead is partially broken. After the encapsulation process, the tie bars are completely punched out by the punching machine. According to such a lead frame, since the lead frame is punched under a condition in which a part of the border between each tie bar and a lead is broken, the punching pressure required for removing the tie bars from the lead frame can be decreased.
However, even if the above lead frame is used, it is still necessary to cut off the tie bars from the lead frame by the punching machine. Thus, the above problem regarding damage to the leads and/or the package by the punching machine is not completely eliminated.